how to put a tracked model behind a person
It is possible, since blender has fully featured masking capabilities.
Here is a simple setup to show you how it works:



If you were able to shoot against a green screen, use this setup instead:



Note: If you need to feather a certain point on you mask, you can hit Alt-S. If you need to feather the whole mask, add a color ramp node after the mask node, you can then tweak the feathering.